In recent years, in the field of information recording, studies on optical information recording methods has been pursued in various places. Such optical information recording methods have advantages of being able to record and reproduce in a noncontact way and deal with the read-only type, the write-once-read-many type and the rewritable type memory forms and are expected to be widely used as methods capable of realizing an inexpensive large-capacity file.
A large capacity of an optical recording medium for a variety of optical information recording forms (hereinafter, also referred to as an optical disk) has been attained by shortening the wavelength of the laser light being light source used in the optical information recording method and adopting an objective lens having a high numerical aperture to make a spot size on a focal surface small.
For example, in a Compact Disk (CD), the laser light wavelength is 780 nm, the numerical aperture (NA) of an objective lens is 0.45, and the capacity is 650 MB, while in a Digital Versatile Disk Read-Only Memory (DVD-ROM), the laser light wavelength is 650 nm, the NA is 0.6 and the capacity is 4.7 GB.
Furthermore, in an optical disk system of the next generation, attaining of a large capacity has been studied by using an optical disk formed with a thin light transmitting protective film (a cover film) of, for example, 100 μm or so on an optical recording layer, irradiating a laser light for recording/reproducing from the protective film side, making a laser light wavelength to be 450 nm or less, and making the numerical aperture (NA) of an objective lens to be 0.78 or more.
Also, in recent years, development of a rewritable type multilayer optical disk having two optical recording layers by using a phase change type recording material has been pursued. Hereinafter, an optical disk having a plurality of optical recording layers will be also called as a multilayer optical disk, and an optical disk having one optical recording layer will be called as a single layer optical disk, respectively.
The present inventors has engaged in development of a phase change type multilayer optical disk and presented at the Optical Data Storage (ODS) Symposium in 1999 and 2001.
A phase change optical disk, regardless of a single layer optical disk or a multilayer optical disk, requires a process called initialization before shipping to the market.
In a production process of a phase change type optical disk, generally, a film of a phase change type recording material is formed on a substrate made of polycarbonate, etc. by a sputtering apparatus, and in a stage after forming the film called “as-deposited”, a phase state of the phase change type recording material is close to an amorphous state.
In a phase change optical disk, when recording information, the state of the phase change type recording material is required to be the crystalline state. The process of changing the amorphous state right after forming the film to the crystalline state is called as the “initialization process”.
In a currently widely used initialization apparatus, crystallization of an overall optical recording layer is performed by converging a laser light on the optical recording layer to be initialized to heat the phase change type recording material and scanning allover the optical recording layer.
At this time, a beam shape of the laser light converged on the optical recording layer has a beam width of, for example, about 1 μm in the disk rotation direction and about 100 μm in the radius direction.
When initializing the optical recording layer on the multilayer optical disk having a phase change type optical recording layer provided as the first layer from the light incident side by using such an initialization apparatus, as described in the Japanese laid open patent publication No. 2001-250265, it is known that due to light interference generated by unevenness of a thickness of an interlayer between a first optical recording layer and a second optical recording layer, a light intensity change of an initializing light occurs on the first recording layer and causes initialization unevenness.